Apparatus for applying leaves of electron emissive material to metallic cathode bases



Aug. 29, 1961 H. w. ROEBER 2,998,052

APPARATUS FOR APPLYING LEAVES OF' ELECTRON EMISSIVE MATERIAL TO METALLIC CATHODE BASES Filed June 20, 1958 17 Sheets-Sheet 1 ATTORNEY Allg- 29, 1961 H. w ROEBER 2,998,052

APPARATUS FOR APPLYING LEAVES OF ELECTRON EMISSIVE MATERIAL TO METALLIC CATHODE BASES Filed June 20, 1958 17 Sheets-Sheet 2 Heiz/y Woef ATTORNEY Aug. 29, 1961 H. w. ROEBER APPARATUS FOR APPLYING LEAVES 0E ELECTRON EMIssIvE MATERIAL To METALLIC CATHODE BASES 17 Sheets-Sheet 3 Filed June 20, 1958 INVENTOR Heil/2y Woeer BY ATTORNEY ug- 29, 1961 H. w. RoEBr-:R 2,998,052

APPARATUS FOR APPLYING LEAVES OF ELECTRON EMISSIVE MATERIAL To METALLIC cATHonE BASES Filed June 20. 1958 1v sheets-sheet 4 Hill JIM

,26 (III. Y 304 Il fn I N VE NTOR (9&4 [Yew/y )M /2065er BY m/ ATTORNEY Aug. 29, 1961 H. w. ROEBER 2,998,052

APPARATUS FOR APPLYING LEAVES OF ELECTRON EMISSIVE MATERIAL T0 METALLIC CATHODE BASES Filed June 20, 1958 1'7 Sheets-Sheet 5 INVENTOR Hen/f Woeef ATTORNEY Aug. 29, 1961 W, ROEBER 2,998,052

APPARATUS FOR APPLYING LEAVES OF ELECTRON EMISSIVE MATERIAL TO METALLIC CATHODE BASES Filed June 20, 1958 l? Sheets-Sheet 6 JINVENTOR Healy Wfoe er BY fza'/ ATTORNEY Aug. 29, 1961 H. w. RoEBER APPARATUS FOR APPLYING LEAVES OF ELECTRON EMISSIVE MATERIAL TO METALLIC CATHODE BASES 17 Sheets-Sheet '7 Filed June 20, 1958 IIMWMMMI- .MVA

ATTORNEY 2,998,052 IvE Aug. 29, 1961 H. w. ROEBER APPARATUS FOR APPLYING LEAVES OF ELECTRON EMISS MATERIAL To METALLIC cATHoDE BASES 17 Sheets-Sheet 8 Filed June 20, 1958 INVENTOR ATTORNEY Aug. 29, 1961 H. w. ROEBER 2,998,052

APPARATUS FOR APPLYING LEAVES OF ELECTRON EMISSIVE MATERIAL To METALLIC cATHonE BASES 1'7 Sheets-Sheet 9 Filed June 20, 1958 Heiz/y Woeer BY M/ ATTORNEY Aug. 29, 1961 H. w. ROEBER 2,998,052

APPARATUS FOR APPLYING LEAVES OF ELECTRON EMISSIVE MATERIAL To METALLIC cATHoDE BAsEs Filed June 2o, 1958 17 sheets-sheet 1o 17 BY r l ATTORNEY H. W. Rol-:BER 2,998,052

TRON EMIssIvE Aug. 29, 1961 APPARATUS FOR APPLYING LEAVES OF' ELEC MATERIAL TO `METALLIC CATHODE BASES 17 Sheets-Sheet 11 Filed June 20, 1958 INVENTOR gem? Woeef' ATTORNEY Aug. 29, 1961 H. w. ROEBER 2,998,052

APPARATUS FOR APPLYING LEAVES OF ELECTRON EMISSIVE MATERIAL To METALLIC CATHODE BASES Filed June 20, 1958 17 Sheets-Sheet 12 ATTORNEY Aug. 29, 1961 H. w. RoEBER 2,998,052 IVE APPARATUS EOR APPLYING LEAVES OF ELECTRON EMISS MATERIAL TO METALLIC CATHODE BASES 17 Sheets-Sheet 13 Filed June 20, 1958 INVENTOR .Henry Woeer BY ATTORNEY Aug. 29, 1961 H. APPARATUS FOR APPLYING LEAVES OF' ELECTRON EMISSIVE Filed June 2o, 1958 W. ROEBER MATERIAL TO METALLIC CATHODE BASES 17 Sheets-Sheet 14 ,y l All 11H I Aug. 29, 1961 H w. ROEBER 2,998,052

APPARATUS FOR APPLYING LEAVES OF' ELECTRON EMISSIVE MATERIAL T0 METALLIC CATHODE BASES 17 Sheets-Sheet` 15 Filed June 20, 1958 INVENTOR Herz/yWEoeber m/ BY ATTORNEY Aug. 29, 1961 H. w. RoEBER 2,998,052

APPARATUS FOR APPLYING LEAVES OF ELECTRON EMISSIVE MATERIAL To METALLIC cATHom: BASES Filed June 20, 1958 17 Sheets-Sheet 16 "Av-vr a l w. su". m

nrj! V Q INVENTOR 15767223/ 14T/:2095er Y MM U B 7 ATTORNEY Aug 29, 1961 H w ROEBER 2,998,052

APPARATUS FOR APPLYNG LEAVES OF' ELECTRON EMISSIVE MATERIAL TO METALLIC CATHODE BASES Filed June 20, 1958 17 Sheets-Sheet 17 lfT il: r*

MUN

IV f' 1f IN VENTOR Henry Wl'zoeer ATTORNEY 2,998,052 APPARATUS FOR APPLYING LEAVES OF ELEC- TRON EMISSIVE MATERIAL TO METALLIC CArII-IODE BASES Henry William Roeber, Emporium, Pa., assignor, by mesne assignments, to Sylvania Electric Products Inc., Wilmington, Del., a corporation of Delaware Filed June 20, 1958, Ser. No. 743,355 20 Claims. (Cl. 154"1.8)

This invention relates to means for applying leaves to bases, for example, leaves of electron ernissive material to metallic cathode base materials to form cathodes used in the manufacture of electronic tubes.

In the manufacture of electron tubes it is necessary to coat a cathode base with an electron emissive material. In a common form of such tube the emissive material is in the form of a coating about a Ihollow metallic sleeve which may have any cross section e.g., cylindrical, oval or rectangular. 'Ihese coatings in accordance with standard practice are applied by utilizing a spraying technique wherein the sleeves are all aligned and placed in a rack or framework such as disclosed in the patent to Morrs 2,786,444 and passed back and forth in front of nozzles which spray the emissive material on the sleeve, with the rack, at times, reversely facing the nozzles to ensure spraying both sides of the sleeves.

This is a messy operation with much waste of material since only a portion of the spray strikes the cathode sleeves and obviously the coating is not uniform about the sleeves or uniform with respect to separate batches of sleeves Ibeing coated.

Where the emissive material is precast into a uniform strip, it can be formed without waste and the strip can be cut up into leaves of a size just sutcient to wrap around the sleeve once, thus ensuring an even coating around the sleeve.

It is a specific object of this invention to provide automatic means to apply such a precast preformed leaf of emissive cathode material onto a metallic base.

It is a further object of the invention to apply the leaf evenly and in a uniform manner to successive bases by causing the leaf to wrap itself around and adhere to the base after an initial contact between the leaf and base has been attained.

These and other objects will become apparent after reading the following description when taken in conjunction with the accompanying drawings in which:

FIG. 1 is a perspective view of the entire cathode sleeve coating machine.

FIG. 2 is a plan view of ythe turret portion of the machine.

FIG. 3 is an elevation of the machine from the material -feed side of the machine and looking, from the right, at the machine of FIG. 2.

FIG. 4 is a horizontal section taken on the line 4 4 of FIG. 3.

FIG. 5 is a vertical section taken along the line 5-5 of FIG. 2.

FIG. 6 is a side elevation of a portion of the machine showing the mechanism for feeding cathode ernissive material to the machine.

FIG. 7 is a plan view of the same portion of the machine.

FIG, 8 is a vertical section on the line 8 3 of FIG. 7.

FIGS. 9 and l0 show two positions of the emissive material stamping out means together with the shifted positions of related parts.

FIG. 11 is a section taken on the line 11-11 of FIG. 6.

FIG. l2 is a section taken along the line 12-12 of FIG. 6.

Patented Aug. 29, 1961 ice FIG. 13 is a vertical section taken along the line 13- 13 of FIG. 7.

FIG. 14 is a vertical section taken along the line 14 14 of FIG. 7.

FIG. 15 is a side elevation of the cathode sleeve feed mechanism.

FIG. 16 is an enlarged front elevational view of a portion of the cathode sleeve hopper.

FIG. 17 is a vertical section taken along the line 17- 17 of FIG. 16.

FIG. -18 Iis a horizontal section taken along the line 18-18 of FIG. 16.

FIG. 19 is an elevational view of the cathode sleeve pick up fingers as viewed along the line 19-19 of FIG. 15.

FIG. 2O is a horizontal section on the line 20-20 of FIG. 19.

FIG. 2d; is a section taken along the line 21-21 of FIG. 2 showing a fragmentary portion of the solvent applying mechanism and associated finger held sleeve.

FIG. 22. is a vertical section on the line 212-22 of FIG. 21.

FIG. 23 is a diagram view of the solvent tank, pump, and associated ports.

FIG. 24 is a fragmentary plan view of the emissive material supporting turret.

FIG. 25 is a section taken along the line 25-25 of FIG. 24.

FIG. 26 is a section taken along the line 26-26 of F-IG. 24.

FIG. 27 is a vertical section taken along the line 27 27 of FIG. 2.

FIG. 28 is a vertical section taken along the line 28- 2-8 of FIG. 27 showing the emissive material just about to be wrapped about the cathode sleeve.

FIG. 29 is a similar view showing the material in the final stage of wrapping.

FIG. 30I is a `perspective view of the fully wrapped cathode sleeve.

FIG. 3.1 is a plan view of the take-olf conveyor with a top heating unit removed.

FIG. 32 is a section on the line 32-32 of FIG. 31.

FIG. 33 is another section on the line 33-33 of FIG. 31 showing drive mechanism for the conveyor.

FIG. 34 is a perspective view of one of the cathode sleeve receptacles on the conveyor, and

FIG. 35 is a section on the line 35-35 of FIG. 2.

In general, the machine specifically illustrated and which carries out the principles of this invention comprises means for stamping or punching out from a strip of emissive material which has a soluble binder, a leaf of such material, depositing each leaf so stamped out onto a turret, supplying cathode bases or sleeves to a conveyor, orientating the sleeves on the conveyor, picking up the sleeves from olf the conveyor by an indexible carrier, applying a solvent for the binder in the emissive coating to the sleeves while held Vin the carrier, `bringing successive sleeves on the carrier and the successive punched out leaves as they are transported on the turret into registration with each other and effecting wrapping of the leaf about the sleeve on the carrier as each carrier reaches registration position. In addition, the machine also applies a drying gas such as hot drying air to the coated sleeves and places the dried coated sleeves on a delivery conveyor where, if desired, they may be further dried.

Referring to the drawings in greater detail, at 20, see FIGS. `2 and 3, there is a table top or base having upstanding bearings 22 supporting a drive shaft 24 having a pulley 26 xed thereon and driven by any suitable electric motor 27, see FIG. 5, through a Reeves transmission 28, supported below the table top.

The shaft has cams thereon which will be described in due course'and is driven from the transmission through a clutch 29. Also xed on this shaft is a bevel gear 30 meshing with a secondv bevelV gear 32 fast on a cross shaft 34 passing through a support housing 36. The

other end of the cross shaft has aiixed thereto a bevel gear 37 meshing with a bevel gear 38 fixed on a countershaft 40 supported in bearings 42 and parallel with the shaft 24. The counter shaft also has a multiplicity of cams thereon which will be described in due course.

VAttached to the housing, see FIGS. 3, 6, 7 andS is a bracket `44, as by bolts 46, The bracket has a pair of parallel spaced guide rod supports 48 in which are slidable a pair of guide rods 50. -Fixed to the guide rods, as by bolts '1 passing through a two part slide carriage 52 tightly embracing the rods is a slide and forming Vpart Vof the slide is a chute 54, held to the carriage by bolts 56. The lower portion of the slide carriage has a bifurcated downward extension 58 to receive the roller 60 of a lever 62 pivoted on a stub shaft 64 supported by brackets 66 onthe support housing. The other end of the lever rides in a cam track in the side face of a cam 70 fast on the countershaft 40. Thus, as the lever is oscillated by the cam 70, the slide carriage and chute are reciprocated.

Movable with the carriage is a clamping mechanism 72 having a rubber pad base 71, see FIGS. 8, 13 and 14, for intermittently clampingand releasing a strip of electron emissive material 73 as it is placed in the chute. For this purpose there is provided a clamp block 74 iitting in the chute and movable toward the bottom of the chute to clamp the emissive strip therebetween. Then Vas the carriage and chute are shifted in one direction,

the clamp block, strip and chute will move together. On the reverse movement of the chute the clamp block will have been released so that it and the chute can move backwardly independently of the strip and a new purchase can be obtained on the strip for the next advance movement thereof with the chute The emissive strip is fed from a roll 76 of such material rotatably mounted in upstanding arms 78 of a bracket 80 fastened to the movable carriage 52. The clamp block is pivoted on the end of a lever 82 in turn pivoted in parallel extensions 83 on the slide carriage and normally spring pressed to clamping position by a spring 84 reacting between the carriage and the lever. The lever 82 is forced to move to a clamp block releasing position by oscillation of a lever y85 with a wide contact plate y36 engaging a roller 88 on the lever l82. The lever 8S, see FIG. 3, is pivoted in a clevis 90 on the bracket 66 and is worked by a track in the peripheral face of a cam 92 fixed on shaft 40.

Adverting to FIGS. 6 to 13, a short distance in advance of the clamp mechanism just described is a punch and hollow die set 100 to stamp out leaves 101 (see also FIGS. 27 to 30) from the emissive strip. The punch, rectangular in cross section, is indicated at 104 and the corresponding rectangular hollow die is indicated at 106. The punch and die operate through a slot 107 in the bottom wall of the chute 54 and a combined stripper pad and clamp 108 assists in the punching out of a leaf from the emissive strip. In the operation of the punch and die and after the strip has been advanced, the die moves upwardly with its neck portion 110 extending through the slot .107 and with the upper edge of the neck portion flush with the upper surface of the lower wall of the chute as in FIG. 9, or just into engagement with the underside of the emissive strip. The stripper pad then clamps the emissive strip between the pad and the top surface of the neck after which the punch approaches the strip and transfers the punched out leaf from the emissive strip, the end of the punch passing completely through the die and almost onto a leaf carrier turret below the chute. For effecting movement of the punch, theY same is fastened by a screw =112, FIG. 8, in a two part stripper support block 114'in turn fastened to a vertical two 4 part sleeve 116, FIGS. Y6 and 7, slidable on a rectangular guide bar 118. At its lower end the bar s fastened by bolts to a keyed support plate 121 suitably fastened to the housing 36.

The stripper block, see FIGS; 9 and l0, slidably retains .a stripper plunger 122 fastened to the stripper pad V108 and is spring pressed toward the chute by a spring 124 within the block and reacting against the plunger.V A head 126 on -the plunger engaging the block 114 limits advance movementl of the stripper pad. On movement downwardly of the sleeve 11,6 relative to the guide bar 118, the stripper pad first engages the emissive strip to hold the same preparatory to `a punching out operation. At

this time the clamp block 74 is raised and the movable p chute 54 and clamp block retract to enable a new bite to be taken by the clamp and chute on the emissive strip. The clamp block and chute organization and the combination of stripper pad and die work Ain alternation to effect a proper feed of the emissive strip. After the punching out of the leaf from the strip, the sleeve rises but the rise of the stripper pad, by reason of its spring mounting, is delayed, thereby enabling the punch to be withdrawn from the die and strip of emissive material without pulling the strip up with it. Eventually, the head 126 engaging the block 114 raises the pad 108 away from the strip. The die has a small movement downwardly in the transfer of the leaf to the turret below and it is moved downwardly so that the top surface thereof does not interfere with subsequent strip advance.

To reciprocate the punch and stripper pad, the sleeve 116, see FIG. 6, is provided with a pivot pin 128 coupled by a link 130 to a pin 132 on a lever 134 pivoted on the stub shaft 64. The lever 1'34, see FIG. 3, is operated by a cam 136 fast on shaft 40and having a groove in its side face in which a follower on the lever rides.

The chute 54, see IFIGS. l and 2, extends over an inalso reciprocable on lthe guide bar 1,18 and has a pivot pin i140 coupled by a spring telescopic link 142, of conventional form, with a pivot pin 144 on a lever 146 also pivoted on stub shaft 64 and operated by a cam 148, FIG. 3, fixed on the shaft 40. The telescopic link allows for excess movement of parts by the cam after the die itself is arrested in its upward motion.

The chute 54, see FIGS. 1 and 2, extends over an indexible leaf transfer turret and conducts the remaining strip material to a suitable receptacle placed at the end of the chute. To facilitate strip disposal, a ber roll 149, driven by chain gear 151 from the shaft 124, Iis mounted on the forward end of the chute.

The gturret 150, see FIG. 5, is mounted on a vertical axis in the housing 36. It comprises a vertical shaft 152 mounted in antifriction bearings 154 in a sleeve 156 hav# ing an eccentric bore and ixed in the housing at predetermined angular relations thereto to vary the lateral position of the axis of the turret with relation to the housing. To the upper end of the shaft is fixed aleaf transfer disc 1 58 and to the lower en'd of the shaft is fastened an indexer gear bearing index rollers 162 at its bottom. The indexer gear has long gear teeth 164 extending in the direction of the vertical axis of the'turret. Cooperative with the underside of the disc isV a ported disc 166 which will be described in greater detail later on. The turret is indexed by means of a cross over cam 168, see FIG. 4, iixed on the Vshaft 34. The indexing arrangement here illustrated is conventional in the art and is described in the patent to Gardner et al. 2,637,144 to which reference may be had for better understanding of the indexing mechanism. Each time the cam A168 rotates one revolution, therturret isindexe'd one-eighth of a revolution. The upper face of the transfer disc, see FIGS. 1, 2, 24, 25 and 26, is provided with eight pairs of depressions 174 extending radially inward from the periphery, for a purpose to be later described and with a slightly raised land 176 perforated as indicatedat 178 inserted between each pair of depressions. On each of these lands is fitted a plate 179 with two rows of apertures 179A and 179B in communication with the perforations in the lands. When the die 6 is at its lower position adjacent the turret in an indexed position of the turret, suction applied -through the perforations 178 will cause the leaf to move against the face of the turret disc and be held there. To assure proper position of the leaf 101 against the plate 179, the plate is recessed as indicated at 181. At another indexed position of the turret air will be blown through these same perforations.

While the leaf transfer disc just described has an indexing motion `carrying the lands 176 around in an orbital path, the valve disc 166 (FIG. 5) below it is fixed. The valve disc is fixed on a pressure plate 180', both the disc and plate being spring pressed against the underface of the perforated transfer disc 158 by springs 182 reacting between the plate 180 and a ring 1183 secured to the sleeve 156. The ring 183 is provided with keys 184 extending into keyways in the lower portion of the hub of plate 18). The pressure plate surrounds a spacing sleeve 186 on the shaft 152. The disc 166 has an arcuate port 198 (see the dotted lines in FIG. 24 and FlGS. 25 and 26) a little less than 90 in extent and a second closely adjacent port 200 `of limited extent, about 10. When the turret is at positions A and B, suction will be applied through the long arcuate port 198 to all of the perforations 178 yat those positions. At position C a blast of air Under control of a cam 281, see FIG. 5, on the cross shaft 34 controlling a piston in an air chamber will be delivered to the perforations 178 via port 200. Air under pressure and suction are delivered to the ports by suitable ducts 203 and 204, respectively.

At position A the leaf is received from the die 106 and at position C the leaf is blown up against a solvent wetted cathode sleeve 205 wetted with a solvent for the binder in the emissive strip so that the leaf may wrap itself about and adhere to the sleeve. The function orf the air is to bring about contact of a limited area of the leaf with the sleeve, after which the leaf aided by the solvent wraps itself completely about the sleeve.

The sleeves are fed to a position immediately above position `C from a hopper 216, see FIGS. l, 3 and l5 to 18. The hopper has a rather thick downwardly sloping bottom wall 212 along which the cathode sleeves will slide to the delivery end of the hopper. In this particular illustration, the cathode sleeves 205 are seamless hollow elongated bodies which are oval in cross section. To feed the sleeves to the spout in vertically stacked relationship, the front wall 214 of the hopper is vertical and spaced away from the front bevelled edge of the wall 212 yby a distance slightly greater than the minor diameter of the oval. Thus the sleeves will be stacked up one above the other, in horizontal position in the space between the two walls with the major axis of the oyals in a vertical plane. To assist in effecting movement of the Sleeves down through the mouth of the hopper, a vibrating tapper 215, FIG. l, is pro-vided to tap against a side wall of the hopper, the same being operated by an air motor 217 supplied by a line 219 under control of a valve 221 and cam 224 on shaft 24. Beneath the hopper is a reciprocable slide 216 provided with a cross recess 218 of a depth to completely receive a sleeve, major diameter vertical, but wide enough to accommodate the sleeve when the major diameter of the sleeve is horizontal. A gating mechanism is provided to dispense the sleeves from the hopper one by one, this comprising a lug 220 fastened to the slide and a spring leaf 222 secured cantilever fashion to the front wall of the hopper. The lower end of the spring leaf passes through a slot in a gate bar 224 which will lrelease a sleeve 205 when the lug 220 tensions the leaf 222. On the return stroke of the slide, the sleeve 205 below the bar and which is now on top of the slide, will fall into the recess 218 in the slide. The slide is further provided with a longitudinal slot 226 intersecting but not as deep as the cross slot 218.

When the sleeve lies broadside down in the slot 218, with the major axis horizontal, the bottom of the slot 226 is above the sleeve. A dog 228 pivoted on a bracket 230 extending from a side wall of the hopper and pressed downwardly by a spring 232 rides the slot 226 and tend to rotate the sleeve to flat horizontal position as the slide with the sleeve in the cross slot rides under the: dog. To hold the sleeve in an adjusted position, the slide is provided with a suction passageway 234 connected to a suction line 235, see FIG. 2, via a valve 237 operated by a cam 239 on shaft 24. The line 235 communicates with the recess 218 via a port 236. To ltriply ensure the positioning of the sleeve in the recess with the major axis horizontal, the slide is reciprocated to a projected plosition when a sleeve is held in the cross slot slightly beyond -where the sleeve is engaged and rotated by the dog. Then the slide is slightly retracted to again cause the dog to engage the sleeve were it fnot previously rotated, and then the slide is projected to a still farther and sleeve discharge position, again subjecting the sleeve to the action of lthe dog. The three motions of the slide in the ad- Vance of the sleeve and their relative extent are indicated at lst, 2nd and 3rd. The means for reciprocating the slide ycomprises a roller 238, FlG. l5, mounted in the forked end 240 of the slide and a bell-crank lever `242 pivoted in a bracket 244 and having spring controlled jalws 246 straddling the roller 238, the lever being oscillated by a cam 248 xed lon shaft 40 and contoured to give the stated oscillatory motion of the slide. Adjustalble stops 250 and 252 are provided to limit the throw of the slide.

At the delivery position, the cathode sleeve is picked up by a pair of fingers 260 and 261, see FIG. 5, mounted on an upper cathode holder turret 262 which indexes through a number (of positions at one of which the cathode sleeve is picked up from the reciprocable bar, at another of which the cathode sleeve is coated with a solvent for the binder in the electron emissive material, at still another of which the emissive coating leaf is applied to the sleeves and art yet another position the coated sleeve is deposited on a carry olf conveyor. The fingers 268 and 261, constituting a pair to grasp a cathode sleeve are mounted on a bracket 264 readily fastened to the turret gs by centering pin 266, FIG. 15, and bolts 268. There are twelve such pairs of fingers. Each of these pairs of fingers is mounted on a portion of a bracket which overhangs the turret, one finger 26?, see FlGS. 19 and 20, being fixed in a recess 269 in the bracket as by a bolt 270 and the other finger 261 being pivoted in a recess in the bracket as by pivot pin 272.

The lower free ends of the fingers are urged together by a spring 273 under compression lying in recesses in the upper ends of the fingers. The lower end of the movable finger is limited in its approach movement toward the fixed finger by reason of a protulberance 274 thereof engaging the bracket 264. The lower ends of the fingers are shaped to hold a cathode sleeve lengthwise therebetween. With some forms of cathodes it is desirable that the sleeve shall be held with the lower edge of the sleeve below the tips of the fingers as is here illustrated. With `other forms of cathode sleeves it may be desirable to retain the sleeves in recesses 276 pnovided in the opposing faces of the fingers near their lower ends. When the cathode sleeve is held by the ngers, the sleeve may be brought close to the upper face of a perforated plate 179 on the emissive material carrier turret. The cathode sleeve is generally held with the minor diameter of the oval vertical and the fingers when lowered by the tun-et, will bring the vcathode sleeve along a line parallel tio the two lines of holes in plate 17 9, see FIG. 28, but preferably offset from the line midway between the two lines so as to cause the leaf o-f emissive material when it wraps itself about the sleeve to have the joint at the side of the sleeve, as seen in FIG. 29, rather than at the mid top of the sleeve. In the case 

